Pure vs impure functions definition and example

Described below are the important distinctions of Pure and Impure functions.

Pure function

• Returned value depends on the values of the arguments
• Pure functions do not call other systems or applications such as network or database calls
• They do not use other variables besides their arguments
• They are independent and only calculate a new value from its arguments
• Pure functions will always return the same result given the same arguments
• they also do not modify the arguments
• Returns a new object

// Pure functions
function double(x) {
    return x + x;
}

function doubleAll(list) {
    // given an array it will not modify the array it will return a new object
    return list.map(double);
}

Impure function

• Impure functions call some external system or application such as a database or network call
• They reach out to other properties of the system or application
• Modify the arguments passed to function
• Returns same object or modified version of same object

// Impure functions
function double(x) {
    networkCall(x);
    return x + x;
}

function doubleAll(list) {
    for (let i = 0; i < list.length; i++) {
        list[i] = double(list[i]);
    }
    return list;
}